The present invention relates to a photo-conductive-type image pick-up tube and an X-ray image pick-up tube, or more in particular to a method of fabricating an image pick-up tube having a target section suitable as an image pick-up tube with an increased target voltage and the particular target section used therewith.
Generally, a photoconductive-type image pick-up tube or an X-ray image pick-up tube (hereinafter referred to as an "image pick-up tube" collectively) comprises a target section for converting an entrant optical image or an X-ray image into a charge pattern and storing it and a scanning electron beam generation section for reading a stored charge pattern as a signal current, wherein the scanning-side surface potential is balanced with the cathode potential immediately after the target is subjected to the scanning of the electron beam.
An image pick-up tube is usually used with a voltage of 300 to 2,000 V applied to a mesh electrode with respect to a cathode and a target voltage of several volts to several hundred volts to a transparent conductive film. In operation, the surface potential at the scanning side of a scanning region may become higher than the cathode potential by a voltage determined by a signal current and the storage capacity of the target section. With a scanning electron beam attached to the surface, however, the potential thereof begins to drop until it is balanced with the cathode potential immediately after scanning. In the process, a part of the scanning electron beam attaches to the scanning-side surface of the target and causes a signal current, while the remaining part of the electron beam returns to the electron gun side and becomes what is called a return electron beam. A part of this return electron beam is reflected from the electrode wall and is scattered to enter the scanning region of the target or around thereof again.
Outside the scanning region, on the other hand, scanning electron beams do not attach unlike in the scanning region, and therefore the scanning-side surface potential is generally considered to be balanced with the target potential. In normal operation, the secondary electron emission ratio outside the scanning region is maintained at a value less than unity, and therefore upon entrance of scattered electrons or other in-tube stray electrons into an area outside the scanning region, the surface potential thereof tends to change, though slightly, toward the cathode potential. If there occurs a dark current or an photo-current due to the unrequired entrance of light, by contrast, these currents act to increase the surface potential, so that the surface potential outside the effective scanning region tends to increase and be balanced with the target potential again. While the image pick-up tube is in operation, therefore, the scanning-side surface potential outside the scanning region is considered to have two factors of change balanced with each other. Specifically, the two factors for changing the scanning-side surface potential outside the scanning region include the current flowing in a photoconductive film acting to increase the scanning-side surface potential and the scattered electrons attached to the scanning-side surface acting to decrease the scanning-side surface potential. The current in the photoconductive film is adapted to flow therein only when there is a potential difference thereacross, and therefore even if the scanning-side surface potential outside the scanning region is increased by the current flowing in the photoconductive film, it would not increase beyond the target potential. As long as the image pick-up tube is in this way of operation, the scanning-side surface potential outside the scanning region is kept below the target potential, and unless the secondary electron emission ratio at this part exceeds unity, the operation of the image pick-up tube remains stable. The conventional image pick-up tubes are operated in such a condition.
These conventional image pick-up tubes are discussed, for example, in "Image Pick-Up Engineering" by Ninomiya et al., published from Corona in 1975, pp. 109 to 116, IEEE Electron Device Letters, EDL-8, No. 9 (1987) pp. 392 to 394, and "A Collection of Drafts for Speeches Before National Conference of Television Society (1982)", by Kawamura et al., pp. 81 to 82. In these conventional image pick-up tubes, if the scanning-side surface is liable to emit secondary electrons by the scanning electron beam, the above-mentioned normal operation of an image pick-up tube would become impossible. As a means for improving the landing characteristics of the electron beam by reducing the secondary electron emission ratio of the scanning-side surface, therefore, a method has been suggested for forming an electron beam landing layer of porous Sb.sub.2 S.sub.3 on the scanning-side surface of the target by vapor deposition in an inert gas (JP-A-52-40809).
Further, in order to produce an output signal of a high S/N ratio by dampening the spurious signals which otherwise might be generated by an extra return electron beam being reflected on the in-tube electrode and re-entering the target during the scanning of the electron beam in these image pick-up tubes, a method has been disclosed for providing an additional electrode outside the electron beam scanning region on the scanning-side surface of the target (JP-A-61-31349), or separating the transparent conductive film on the light entrance side of the target into two parts corresponding to the effective scanning region of the electron beam and the remaining region, which are controlled by being connected to independent power supplies respectively (JP-A-63-72037).
If the photoconductive film of the target section is to be thickened for improving the sensitivity or reducing the capacitive lag or if an avalanche multiplication is to be caused in the photoconductive film for further improving the sensitivity of these conventional image pick-up tubes, it is necessary to increase the voltage between the target electrode and the cathode of the image pick-up tube (hereinafter referred to as "the target voltage").
With the increase in target voltage, however, the impinging energy of scattered electrons is increased, increasing the secondary electron emission ratio to more than unity, with the result that the scanning-side surface potential outside the scanning region would begin to increase beyond the target potential. This increase in surface potential, which further facilitates the emission of secondary electrons, would steadily increase the scanning-side surface potential outside the scanning region until it is balanced with an electrode potential higher than the target potential, for example, a mesh electrode potential. An increase in the scanning-side surface potential outside the scanning region would affect the track of a scanning electron beam scanning the peripheral parts of the scanning region, thereby preventing the scanning electron beam from entering the target in perpendicular direction. As a consequence, the emission of secondary electrons by the scanning electron beam would be increased around the scanning region, and the resulting unstable scanning would cause what is called "the waterfall effect" on the screen or an inversion phenomenon by transfer to a high-speed scanning.
As mentioned above, the phenomenon of waterfall or inversion is a fault attributable to the unstable operation caused by the increase in the scanning-side surface potential outside the effective scanning region which will occur in the case where the target voltage or mesh voltage is increased in operation.
The cause of the waterfall phenomenon will be described more in detail with reference to the partly cutaway sectional view of the parts in the vicinity of the target section of an image pick-up tube shown in FIG. 1A and a photoconductive film and the diagram of surface potential distribution thereof shown in FIG. 1B The surface potential within the scanning region is rendered substantially equal to the cathode potential by the scanning beam as shown in FIG. 1B. The potential outside the scanning region, on the other hand, is equal to the target voltage applied to. When stray electrons (called return beam) enter the area outside the scanning region and the surface secondary electron emission ratio exceeds unity as compared with the value within the scanning region, the surface potential thereof gradually increases under the effect of the voltage of the mesh electrode in the vicinity thereof until finally it is balanced with the mesh voltage. As a result, the surface potential is increased from the original level, thus further aggravating the waterfall.
In order to solve this problem, the entire area of the film on the electron beam scanning side of the target section may be composed of a material high in the degree of porosity as shown in FIG. 2A or the thickness of the Sb.sub.2 S.sub.3 may be increased, thus eliminating the waterfall phenomenon as is clear from the mark .DELTA. in FIG. 2B, although the lag characteristic is deteriorated.
As will be seen from the foregoing description, the use of an image pick-up tube with a high target voltage is liable to cause an abnormal pattern changing like a ripple around the reproduction screen of the monitor (hereinafter referred to merely as "the waterfall phenomenon") or the polarity inversion of a signal output of an image pick-up tube at a part corresponding to the peripheral parts of the screen (hereinafter referred to merely as "the inversion phenomenon"), thereby making it impossible to obtain a satisfactory image. A conventional method for dampening the generation of these faulty phenomena has been by increasing the degree of porosity or film thickness of an image pick-up tube having an electron beam landing layer of porous Sb.sub.2 S.sub.3. This method, however, has a disadvantage in that the resistance of the electron beam landing layer increases resulting in an increased lag.